Insects And Evolution

Editor’s Note: This is a guest post by post-doctoral researcher Michelle Trautwein, whose work at NC State focuses on entomology, genetics and evolution. Here she explains what we know about insect evolution – and why it matters.

Our planet is swarming with insects. Literally. Six-legged creatures account for the majority of life on earth, by far outnumbering plants, fungi or anything with a backbone.  Though small in size, they play a huge role in human society as disease vectors, agricultural pests, and model organisms critical to discoveries in health and science.  Terrestrial ecosystems everywhere are ruled by insect pollinators, predators, parasites and decomposers. In short, insects are important in and of themselves.

But insects are also important because they can offer us insight into how life has developed more broadly – understanding insect evolution is key to understanding the evolution of life generally. Yet the evolutionary relationships of insects are still largely unknown despite decades of research aimed at resolving them. To better understand how much we know – and how much we have left to uncover – I am part of a team that authored a new review that takes a look at the recent successes (and failures) in recovering the insect tree of life and presents a general consensus of what we know about these relationships today.

One of the most exciting recent findings in insect evolution is that insects are actually crustaceans. This may not seem surprising, given the superficial similarity between, say, insects and shrimp – the exoskeleton, the many bendy legs – yet insects have long been considered the closest relatives to the millipedes and centipedes.  Turns out the traits shared by insects and millipedes and centipedes are simply adaptations for living on land and have evolved independently in both groups. Though insects have reigned over the land and air, they have never invaded the sea. Now this new evolutionary context makes it clear that insects are the terrestrial branch of a primarily aquatic group, and together these crustacean lineages have conquered both land and water.

A persistent puzzle in the insect tree of life is the evolutionary sequence of some of the earliest branches. The very first insect lineages are the earth-bound silverfish and their relatives. Next up emerges insects’ finest innovation: wings. Both dragonflies and mayflies are currently vying for position as the first winged insects. Or maybe it was both of them together? The wings of dragonflies and mayflies aren’t very flexible and, unlike the wings of all other insects, they can’t be folded across their backs. This shared trait between mayflies and dragonflies has long united them in a group called Palaeoptera, or “old wings.”  Yet this is an area of the insect tree of life that scientists can’t seem to agree on. Recent molecular studies have only added to the debate with conflicting results that support every competing hypotheses for the relationships of the early winged insects.

Over 300 million years  ago, many new forms of insects emerged with a bang in rapid succession. Grasshoppers, earwigs, walkingsticks – along with less well-known groups such as stoneflies, zorapterans, icecrawlers and webspinners – all diversified so quickly (well, quickly from an evolutionary standpoint…over the course of 50 million years) that little genetic trace is left to reveal how they are each related to the other. Yet recent studies have solved one mystery – the evolutionary relationships of Dictyoptera – a group that includes cockroaches, termites and praying mantises. Indeed, it now appears that cockroaches and termites are so closely related that termites themselves are actually just social, wood-eating roaches.

Another important success has been resolution of the relationships of insects that undergo complete metamorphosis (with larval, pupal, and adult stages – called Holometabola).  Holometabola includes the four superradiations that make insects truly diverse: wasps, beetles, moths and flies. We now know that  wasps (along with bees and ants) were the first holometabolan lineage to evolve.

Another holometabolan mystery recently solved is the placement of the bizarre, insect parasite order called Strepsiptera. Strepsipterans are not at all uncommon, though you likely have never seen one. The legless females spend their whole lives inside the body of another insect host, while the tiny, winged males have a brief life in the outside world. For ages, it was completely unclear who strepsipterans were related to.

While some scientists considered them beetle relatives, a controversial hypothesis based on molecular data and a dubious interpretation of wing homology placed them as the closest relatives to flies. Heated debates persisted in the insect science world, yet no convincing evidence emerged for over a decade. Finally, a growing body of molecular and morphological data firmly places strepsipterans as close beetle relatives.

Much of insect evolution is still shrouded in mystery, particularly the relationships of many common groups that rapidly diversified over 300 million years ago. Yet this new review celebrates recent successes and provides a framework of where insect relationships stand today that will allow us to judge the incoming wave of molecular data that awaits us in this genomic era.

The review paper, “Advances in insect phylogeny at the dawn of the postgenomic era,” is published in the 2012 Annual Review of Entomology, and was co-authored by Trautwein, Brian M. Wiegmann, Rolf Beutel, Karl M. Kjer, and David K. Yeates.